Phosphorus is both a plant and animal nutrient and an environmental contaminant in the modern world, implicated as a major source of eutrophication of surface waters. Both urban and agricultural waste streams contain phosphorus that entered the element cycle as a nutrient but that is difficult to remove and recover in a recyclable form and, therefore, is more nuisance than nutrient. Sewage treatments plants are obliged to reduce phosphorus levels in discharge water to low levels, but typically do so by directing the phosphorus to the sewage sludge, or biosolids, which are usually land applied. In doing so, sewage treatment plants are often faced with nuisance formation of phosphate minerals, principally struvite, in pipes, heat exchangers, and tanks due to the high levels of phosphate produced during anaerobic digestion of the solids. The biosolids have an unfavorably high phosphorus/nitrogen ratio, so that if biosolids are land-applied to meet crop nitrogen needs then the added phosphorus exceeds crop needs and will either accumulate to undesirable levels in the soil or be prone to runoff losses with erosion. Rock phosphate mineral resources for fertilizer production are nonrenewable and limited to another century at current rates of use.
The production of biosolids in sewage treatment plants employs anaerobic digestion of a combination of two feedstocks: first, primary sludge produced by produced by settling and grit removal of the raw sewage, and second, waste activated sludge produced from the treated water by biological nutrient removal using polyphosphate accumulating organisms to accumulate phosphorus in their biomass before wasting to be anaerobically digested as part of the biosolids.
A number of methods have been devised to recover phosphorus from sewage treatment plants. There are basically two processes to recover phosphorus from wastewater using crystallization reactions: the hydroxylapatite process and the magnesium ammonium phosphate process. In the hydroxylapatite process, a calcium source is added into the wastewater and phosphorus is recovered in the form of calcium phosphate (Hirasawa et al, 1981a, 1981b, 1981c). In the magnesium ammonium phosphate process (Ohlinger et al., 1998; Durrant et al., 1999; Shimamura et al., 2003; Yoshino et al, 2003), a magnesium source is added (often as magnesium chloride), sometimes with the addition of base (sodium hydroxide or magnesium oxide) to raise pH of the water treatment, and the phosphorus is recovered in the form of magnesium ammonium phosphate hexahydrate, i.e., struvite.
Such methods for phosphorus recovery have been proposed to be located either before or after the anaerobic digestion of primary sludge and waste activated sludge. FIG. 1, points 1 and 2, shows the location of such P removal methods in a typical wastewater treatment plant. U.S. Pat. No. 7,182,872 (Barak, et al., 2007), as well as Jaffer et al. (2002), Britton et al. (2005), Bhuiyan et al. (2008) and Le Corre et al. (2007), considered the filtrate or the centrate of the anaerobic digesters at sewage treatment plants, as in such as FIG. 1, point 2, as the most promising spot for struvite formation, with the primary deficiency—insufficient magnesium concentration in the filtrate—to be addressed by addition of magnesium chloride (or magnesium-saturated cation exchange resins, per Barak (2007) and pH to be adjusted upward if needed by addition of sodium hydroxide. Accordingly, Barak, et al. (2007) discloses a method and apparatus for removing phosphorus as struvite from filtrate or centrate from anaerobic digester of a sewage treatment plant by means of a negatively-charged compressed monolayer, self-assembled monolayer, or polymeric cation exchange membrane. Alternatively, U.S. Pat. No. 6,338,799 (Fukushima, et al.) discloses a method for recovering phosphate from waste activated sludge in a phosphorus-releasing tank before anaerobic digestion, such as FIG. 1, point 1. The process includes treating sludge drawn from a water treatment system at a sewage treatment plant in an anaerobic condition to release polyphosphate accumulated in the sludge into solution, and recovering phosphate in the solution using a seed crystal material. The system for recovering phosphate from sludge includes a phosphorus-releasing means for treating sludge drawn from a water treatment system at a sewage treatment plant in an anaerobic condition to release phosphate into the bulk liquid, a dewatering and separating means for separating the sludge containing the solution including the released phosphate into dewatering effluent and dewatered sludge, a calcium ion concentration-adjusting means for adjusting the calcium ion concentration in the dewatering effluent, a means for adjusting the pH of the dewatering effluent to pH 7.5 to 9, and a crystallizing means for recovering phosphate from the calcium ion concentration-adjusted, pH-adjusted effluent of dewatering apparatus.
In recent years, multi-phase anaerobic digesters have been introduced that entail a sequence of organic acid digester, thermophilic digester, and mesophilic digester. FIG. 1 (bottom) shows such a multiphase anaerobic digester. The purpose of such an arrangement is to optimize environmental conditions for the several microbial processes involved in anaerobic digestion and thereby enhance methane production in the thermophilic phase and produce biosolids with reduced pathogen content. Key to the multi-phase process is the organic acid digester, which produces low molecular weight organic acids from digestible carbohydrates at mesophilic temperatures by processes of acidogenesis and acetogenesis, with a retention time of several days. The organic acid digest then passes to the thermophilic digester where, at higher temperature and higher pH, the microbial process of methanogenesis produces methane in the form of biogas.